The use of photoinitiator systems to initiate photopolymerization is well known. When irradiated by actinic radiation, the photoinitiator system generates free radicals, which initiate polymerization of the monomer or monomers. The photoinitiator system may be a single compound which absorbs actinic radiation and forms the initiating radicals, or it may consist of several different materials which undergo a complex series of reactions to produce radicals. Added components, which do not absorb actinic radiation, but which increase the efficiency of the photoinitiator system, are known as coinitiators.
Many of the well-known photoinitiator systems limit the applicability of photopolymerizable compositions because they are activated only by ultraviolet radiation. The availability of reliable, relatively inexpensive, computer controlled visible lasers, which can expose the photopolymer directly, eliminating the intermediate photomask, has made it essential that initiator systems which can be activated by visible radiation be developed.
Applications for photopolymerizable compositions which are sensitive to visible radiation include: graphic arts films, proofing, printing plates, photoresists, and solder masks. The preparation of holograms in photopolymerizable compositions, such as are disclosed in Haugh, U.S. Pat. No. 3,658,526; Keys, U.S. Pat. No. 4,942,102; Monroe, U.S. Pat. No. 4,942,112; Smothers, U.S. Pat. No. 4,959,284; and Trout, U.S. Pat. No. 4,963,471, also require initiator systems which can be activated by visible radiation.
A large number of free-radical generating systems have be used as visible sensitizers for photopolymerizable compositions. Photoreducible dyes and with various coinitiators have been widely studied. A useful discussion of dye sensitized photopolymerization can be found in "Dye Sensitized Photopolymerization" by D. F. Eaton in Adv. in Photochemistry, Vol. 13, D. H. Volman, G. S. Hammond, and K. Gollinick, eds., Wiley-Interscience, New York, 1986, pp 427-487. Many dye sensitized systems undergo dark reactions and are not shelf stable.
Shelf stable initiator systems containing borate anion coinitiators have been disclosed in Gottschalk, U.S. Pat. Nos. 4,772,530 and 4,772,541. The photopolymerizable compositions contained a cationic dye-borate anion complex which is capable of absorbing actinic radiation and producing free radicals. Cationic methine, polymethine, triarylmethane, indoline, thiazine, xanthene, oxazine, and acridine dyes were disclosed. Triaryl alkyl borate anions were the preferred borate coinitiators. Additional photopolymerizable compositions containing a salt formed by an organic cationic dye compound and a borate anion are disclosed in Yamaguchi, U.S. Pat. Nos. 4,902,604 and 4,952,480. Initiator systems containing a transition metal complex and a borate ion are disclosed in Adair, U.S. Pat. No. 4,954,414.
Koike, U.S. Pat. No. 4,950,581, discloses photopolymerizable compositions containing an organic dye and a triaryl butyl borate anion coinitiator. In these systems the dye was non-ionic. Merocyanine type dyes, coumarin type dyes, and xanthene and thioxanthene dyes were disclosed.
Initiator systems comprising a squarylium dye and a halomethyl triazine are disclosed in Nagasaka, EPO Application 90/101,025. These systems were sensitive to light having a wavelength of 650 nm.
Despite the advances which have been made, there is a continuing need for improved initiator systems with high sensitivity to the red region of the visible spectrum.